The present invention relates generally to geophysical exploration and more particularly to methods of acquiring and processing seismic data to obtain improved structural images of the earth's subsurface formations.
Seismic exploration techniques generally involve imparting seismic energy into the earth and recording the earth's response thereto. Typically, seismic energy can be imparted into the earth using seismic sources or explosive charges. The seismic energy refracted, reflected and scattered by the earth can be detected by seismic receivers to produce seismic signals or traces which explorationists can use to evaluate the earth's subsurface formations.
Most seismic exploration techniques employ colinear arrays of shot points and receiver locations for imparting seismic energy into the earth and recording the earth's response thereto. Such seismic exploration techniques are conventionally referred to as two-dimensional (2-D) methods since they assume that all of the events in the recorded seismic signal come from a vertical plane defined by a seismic line of profile (A) containing the linear arrays of shot points (S) and receiver locations (R) as shown in FIG. 1. Since the earth's subsurface structure is generally not two-dimensional, but rather three-dimensional, the recorded seismic signals can contain reflection events from all directions, including those in the vertical plane defined by the seismic line of profile A. Although out-of-the-plane events can oftentimes be recognized by experienced seismic interpreters, these out-of-the-plane events can cause the seismic data to be misinterpreted.
FIG. 1 also depicts a dipping plane interface P in the earth. Line of profile A coincides with the strike direction, line of profile B coincides with the dip direction of the plane, and line of profile C is an arbitrarily oriented seismic line of profile. Conventional 2-D seismic processing of data along each line includes normal move-out correction using only estimates of seismic velocities. However, the normal moveout correction is also dependent upon the dip and azimuthal orientation of the dip for the dipping plane P. Consequently, for an arbitrarily oriented dipping surface, 2-D data and 2-D normal moveout corrections can at best only compensate for apparent dip in the inline direction, (i.e., parallel to the line of profile.) As such, an incorrect estimate of the subsurface velocities is obtained, as well as an incorrect structural image of the earth's subsurface.
A first approach to ameliorating the effect of dipping interfaces is to orient the seismic line of profile so as to coincide with the dip direction. Aligning the seismic line of profile along the dip direction can eliminate recording out-of-the-plane reflection events. However, aligning the seismic line of profile in the dip direction is not always possible because of the generally complex and varying geological structure found in certain locations. Alternatively, three-dimensional (3-D) seismic acquisition and processing techniques have been developed to overcome some of the shortcomings of the 2-D seismic acquisition techniques. Rather than employing a colinear array of shot points and receiver locations, as in the 2-D technique, 3-D seismic acquisition techniques employ areal arrays of shot points and receiver locations as described in U.S. Pat. No. 4,001,770 by Hofer and in U.S. Pat. No. 4,403,312 by Thomson. Additionally, 3-D processing techniques employing a three parameter normal moveout correction which advantageously integrates the variations of dip angle, velocity and azimuthal orientation of each source receiver pair with respect to the dip direction can be applied to 3-D seismic data to obtain better structural images of the earth's subsurface. Additional 3-D processing techniques can be employed to estimate both dip and dip azimuth of reflecting interfaces in the earth's subsurface. Because of the areal extent of 3-D seismic surveying techniques, the 3-D seismic acquisition techniques tend to be much more expensive than 2-D seismic acquisition techniques.
As a compromise between the inexpensive nature of 2-D seismic surveys and the more costly 3-D seismic surveying techniques, an alternative seismic acquisition and processing technique has evolved employing either a linear array of shot points and a limited two-dimensional array of receiver locations typically comprising several parallel, linear receiver arrays or a linear array of receiver locations and a limited two-dimensional array of source locations. Such techniques are generally referred to as wideline profiling. Exemplary of which, is the seismic exploration technique described in U.S. Pat. No. 3,838,390 by Michon. However, seismic data acquired using wideline profiling techniques are processed as conventional 2-D seismic data. Two-dimensional processing techniques are used with such wideline seismic data because the wideline profiling techniques simply do not obtain seismic signals for a broad enough range of source receiver pair azimuthal orientations to be effective with 3-D processing techniques. Moreover, wideline seismic data cannot be employed to estimate either dip or dip azimuth so as to obtain true dip corrected move-out corrections. Consequently, wideline profiling techniques can still yield seismic data which can be difficult to properly interpret in areas having complex 3-D geological features.
In order to overcome these limitations, the present invention describes a novel scheme for collecting seismic data by positioning shot points and receiver locations whereby the collected seismic data resulting therefrom provides a broad range of source-receiver pair azimuths which can effectively be processed employing 3-D processing techniques. This enables direct measurement and correction for dip and dip azimuth to be made in the normal moveout correction. These and other benefits of the present invention will be apparent from the discussion below with reference to figures in the included drawings.